Temperature stabilized spectral source

ABSTRACT

A temperature stabilized spectral source comprising an electrodeless discharge lamp energized in a radio frequency field, the temperature of the lamp being controlled by disposing the lamp in air or other gas of controlled temperature. The exciting field is typically in the microwave frequency range.

nited States Patent 91 Browner et a1.

[ Jan. 15, 1974 1 TEMPERATURE STABILIZED SPECTRAL SOURCE [75] Inventors: Richard F. Browner; James D.

Winefordner, both of Gainesville, Fla.; Tom H. Glenn, deceased, late of Gainesville, Fla. by Mary T. Glenn, heir [73] Assignee: Board of Regents, Tallahassee, Fla.

by said Browner and Winefordner [22] Filed: Mar. 6, 1972 [21] Appl. No.2 232,035

[52] U.S. Cl 315/248, 313/44, 315/50, 315/115, 356/85 [51] Int. Cl.. G03b 27/04, HOSb 41/16, HOSb 41/24 [58] Field of Search 315/50, 248, 115; 313/44, 47; 356/85, 87

[56] References Cited UNITED STATES PATENTS 2,975,330 3/1961 Bloom et al. 315/248 QUARTZ JACKET ELECTRODELESS DISCHARGE LAMP "AHTYPE ANTENNA OTHER PUBLICATIONS D. O. Cooke, R. M. Dagnall, and T. S. West, Evaluation of Some Three-Quarter-Wave Microwave Cavities for the Operation of Electrodeless Discharge Lamps, Analytica Chimica ATA, Vol. 56, 1971, pp. 17-28.

Primary Examiner-Roy Lake Assistant Examiner-Hugh D. Jaeger Att0rney-George l-l. Baldwin et a1.

[57] ABSTRACT A temperature stabilized spectral source comprising an electrodeless discharge lamp energized in a radio frequency field, the temperature of the lamp being controlled by disposing the lamp in air or other gas of controlled temperature. The exciting field is typically in the microwave frequency range.

14 Claims, 5 Drawing Figures HEATER coll.

PATENIEUJAN 1 51974 SHEET 1 (IF 4 COIL n 'HEAT ER ELECTRODELESS DISCHARGE LAMP T E K C A J Z T R A U Q \METAL HOLDER yam. QUARTZ ENVELOPE PAI'ENIEBJIN 1 SW 3, 786.308 SIIEU 3 0f, 4

IIIIIII RADIANT OUTPUT 6 (ARB.UNITS) I IIIIIIII IIIIIIIII I I IIIIIII SPECTRAL I l I I .20 40 so so I00 I20 I40 MICROWAVE POWER(W)..

PAYENTE JAN 1 51974 *sawwr r:h b

w w w w TEMPERATURE STABILIZED SPECTRAL SOURCE Electrodeless discharge lamps comprising a sealed off quartz tube envelope, typically of about I centimeter in external diameter and 4 or 5 centimeters in overall length, and containing an inert gas, such as argon or helium, and an excitable substance, such as mercury, zinc, thallium, sulfur, copper, cadmium or other element, or a halide of one of them, have been known and used to produce spectral sources useful, for example, in spectrographically examining specimens for atomic absorption and atomic fluorescence. A lamp of this type is commonly known as an EDL. Such lamps are widely recognized to possess many advantages over other types of high intensity, narrow-line atomic sources, being characterized by high spectral radiance and purity of the radiation, together with simplicity of construction and low unit cost.

In spite of the proven ability of EDLs to provide low limits of detection in atomic fluorescence spectrometry and high sensitivity in atomic absorption spectrometry, their use is still generally restricted to research laboratories due to the high degree of operator skill and experience which is necessary in order to approach optimum performance from the sources. For example, even though a particular source may be intense and stable and may possess a reproducible output upon reinitiation, it is unusual to obtain the identical performance from another lamp containing the same excitable element, and even the same lamp may not behave identically if it is removed from the microwave field and then replaced in the field and re-initiated. Moreover, lamps containing certain elements, e.g. copper, become difficult to re-initiate after several hours of operation due to the deposition of a metallic film on the walls of the quartz envelope, since the film shields the gaseous substances in the envelope from the microwave field and reduces the efficiency of excitation of the discharge.

There are a number of operating parameters which have been recognized as influencing EDL performance, such as the nature and pressure of the fill gas; the choice of fill material, which may be one or another excitable element, or combinations of excitable elements, or halides thereof; the configuration and dimensions of the lamp envelope; the nature and characteristics of the microwave energy coupling device, whether an antenna or tuned cavity; and the frequency and intensity of the radio frequency or microwave energy. The desirability of heating discharge lamps in which excitation is provided by a microwave field has been previously noted, and various suggestions and procedures for so doing have been attempted with only partial success. For example, hollow cathode devices and current heated carbon or iridium wire filament devices, and devices comprising one or another type of heating element at the lamp with the heating element aran'ged to become hot by absorption of a part of the available microwave energy have been developed or suggested. Metallic members disposed adjacent or within a discharge device which'it is desired to excite by deposition in a microwave field, however, absorb energy from the field and tend to shield the substance to be excited from the microwave energy. Furthermore, it is very difficult to control such arrangements to provide the desired operating temperature.

It has been determined that the temperature of an EDL is critical to obtain maximum spectral radiance and stable operation, and reproducible results, in spectral analysis and for similar purposes. The arrangement shown herein is simple and inexpensive and provides for heating of the EDL without absorption or interference with the microwave field, and permits operation of a high intensity spectral source under optimum conditions by personnel having only limited technical training, whereas prior art arrangements have required extensive preparatory setting up by highly trained scientists, and even when so set up, have been unstable and of restricted dependability.

The novel features which are believed to be characteristic of this invention are set forth with particularity in the appended claims. The invention itself, however, both as to its organization and method of operation, together with further objects and advantages thereof, may best be understood by reference to the following description taken in connection with the accompanying drawing, in which:

FIG. 1 is a diagrammatic representation of a temperature stabilized spectral source according to the invention;

FIG. 2 is a side elevational view of an EDL mounted in a holder as employed in the arrangement of FIG. 1;

FIG. 3 is a chart showing absorbance, by the atoms in a specimen, of radiation from a spectral source in accord with the invention and embodying an EDL containing the respective excited metallic atoms, as measured by atomic absorption spectrometry, plotted against EDL temperatures;

FIG. 4 is a chart showing typical spectral radiant output for a cadmium EDL maintained at substantially 250 C. (upper curve) and of the same EDL excited in microwave fields of various powers disposed in ambient air and heated only by the effects of the microwave fields (lower curve); and

FIG. 5 is a chart showing the spectral radiant output of EDLs containing various metals plotted against the temperature of the EDL.

In obtaining the experimental results represented by the tables and charts, EDLs were used containing mercury (Hg) as a metal, cadmium (Cd) as a metal, zinc (Zn) as a metal, thallium (Tl) as a thallium iodide, iron (Fe) as ferrous chloride, and copper (Cu) as cuprous chloride. The lamps were filled with argon at 2 Torr pressure except that the thallium lamp was sealed at 5 Torr pressure. While the inert gas in the EDLs used to obtain the outlined results was argon, helium may be preferred. EDLs of the type employed have been known in the art.

Referring now to FIG. 1, EDL l is disposed in a hollow cylindrical quartz jacket 2 having an internal diameter of about three times the outer diameter of the EDL envelope and having a length preferably at least about as long as, and typically about one and one-half times as long as, the length of the EDL. The quartz jacket 2 is open at opposite ends so that air introduced into the open upper end 3 will pass through the jacket and out through the open bottom end 4.

While the quartz jacket 2 may be unbroken if EDLs are to be employed as sources of light of less than about 2,000 A, it is preferred, particularly for light of shorter wavelengths, to provide a small window opening 5 in the side of the jacket to permit light from the source to reach the materials being tested without the absorption which is characteristic of even high purity quartz. The open window in the quartz jacket 2 may be about 3 centimeters long and l or 2 centimeters wide. It will be understood that the dimensions given hereinabove are exemplary only and are not critical.

The quartz jacket 2 is shown as supported at its upper end 3 by a throat portion 6 of a metal air duct 7. Immediately above the quartz jacket the duct extends outwardly in a generally horizontal direction, that is to say, in a direction which is away from the microwave energy field. Within a portion 8 of the duct which is well away from the microwave field, there is disposed a heat exchange means or device, in the drawing represented as a heater coil 9. The heater coil 9 may comprise a nichrome wire electrical resistance element which may be typically of about 2,000 watts rating. A means in the form of a fan 10 is provided to introduce ambient air into the end 11 of the duct 7 which is remote from the quartz jacket 2. The air from the fan 10 passes through the heat exchange device 9 and is conducted by the duct 7 into jacket 2. A microwave antenna 12 is disposed immediately below the lower end of the quartz jacket 2 but is spaced sufficiently therefrom to permit the free escape of air from the end 4 of the jacket. A frame 13 supports the antenna 12 and the duct structure 7 in rigidly maintained relative positions. The duct structure is preferably wrapped with asbestos insulation or the like to minimize the heat loss, although such insulation is omitted from the drawing for clarity. While air is conveniently used, other gaseous mediums may be employed if desired.

The fan 10 typically passes a few liters of air per minute into the duct 7 and thence through the jacket 2, such as between about 1 and 20 liters per minute.

While the fan 10 is shown as merely supplying ambient air into the duct 7, it may be preferred to collect the air issuing from the quartz jacket 2 and to conduct this air into the fan input thereby to provide recirculation. The air from the quartz jacket 2 may, for example, be conducted by a quartz duct joined to the bottom of the jacket, quartz to avoid interference in the microwave field, and this duct, in turn, would be connected to a less expensive duct section leading to the fan input. Such an arrangement would reduce the heating requirements imposed on the heater coil 9 and would provide somewhat closer control of the temperature but would add to the complexity and cost of the device. If the open window 5 is provided in the jacket 2, such recirculation would be less effective since a certain portion of the air escapes through the window. The portion so escaping has, in practice, proved to be very small.

The microwave antenna 12 which is illustrated in FIG. 1 comprises an A type of antenna which focuses the electromagnetic microwave energy at substantially the center of the jacket where the EDL is disposed. The microwave energy source represented generally at 14 which supplies the antenna 12 may be a 120 watt, 2,450MHz generator.

It has been found that EDLs may be excited effectively by being placed in a microwave field focused at the EDL by an antenna 12 as shown. A typical A type antenna suitable for use as antenna 12 is model 2254- 5002Gl and a suitable generator for driving such antenna is model PGM-IO, each as made and sold by the Raytheon Co., Microwave Devices, Farmington, Conn.

Referring again to FIG. 1, a chromel/alumel thermocouple junction 15 is located in the air stream between the heater coil 9 and the EDL, and preferably in or immediately adjacent throat portion 6 of the air duct 7. The thermocouple 15 is connected directly to a temperature controlling device represented at 16, which may comprise a Barber-Colman Model 272? temperature controlling device as manufactured by Barber- Colman Company, Rockford, Ill., and the thermocouple 15 and controlling device maintain the temperature of the air stream at the thermocouple at the desired preset value by regulating the current to the heater coil 9. Alternatively, and preferably, to provide more convenient and stable control, a proportional controller may be substituted for the above less expensive Model 272?.

A threaded opening 17 is provided in duct 7 directly above the desired location of the EDL, and as shown in FIG. 2 the EDL envelope is connected, such as by means of a high-temperature-resistant cement, to an externally threaded metal holder in the form of supporting rod 18 having screw threads 19. The desired EDL is placed in the apparatus by being inserted through opening 17 and by then screwing the metal holder 18 into the opening. The EDL may be precisely adjusted in elevation with respect to the antenna 12, and may thus be located at the focus of the antenna, by screwing the metal holder 18 in and out.

While the jacket 2 has been described as preferably being of quartz, and quartz has the desirable characteristic of absorbing very little of the energy in a microwave field, and the further desirable property of passing light waves with minimal absorption, it will be understood that a ceramic jacket would be a suitable replacement provided that a window 5 is to be employed, it being only necessary that the jacket should be able to withstand the temperatures to be employed and should not be of a material which will provide more than minimal shielding or absorption effects in the microwave field.

Microwave power is difficult to control accurately, and precise positioning of an EDL in a microwave field is difficult and time consuming, and if, according to the prior art, it is also necessary to precisely locate a heater so as to absorb a part of the microwave energy, the positioning of the heater is similarly critical. The positioning of the heater determines the amount of heat which the heater will produce. It is almost impossible to define exactly where a specific lamp and a specific microwave powered heater would have to be located in any particular microwave field environment. It has been found that two important advantages accrue from the arrangement according to this invention in that, first of all, by carefully controlling the temperature of the EDL, the positioning of the EDL in the microwave field becomes much less critical and the control of the intensity of the field is likewise much less critical. As will be seen in curve (a) of FIG. 4, which is a curve applicable to a cadmium EDL maintained in the environment of FIG. 1, and in which the thermocouple junction is maintained at 250 C, and whereby the EDL is therefore maintained at substantially this temperature or a very few degrees below this temperature, the radiant output of the EDL is substantially constant for microwave power varying from about 40 watts to watts. Accordingly, displacement of the EDL from the optimum position in the microwave field, which has the effect of changing the field intensity at the EDL, or a change in the absolute field intensity, has very little effect on the radiant output.

It has been determined to be impractical simply to increase the microwave field strength to attempt to cause the EDL to heat itself to a desirable operating temperature. The optimum operating temperature for a Cd EDL, for example, for atomic fluorescence (A.F.) spectrometry, has been found to be approximately 250 C. and for atomic absorption (A.A.) spectrometry approximately l35 (3., as shown in Tables I and 2 set forth hereinafter and in FIG. 3, whereas it will be seen from FIG. 4, curve (b), that, even with a microwave generator power of l25 watts, the EDL being heated only by the microwave energy, i.e. with the heating coil and fan inoperative, the EDL reaches only 145 C, while at 250 C, with the EDL heated in accord with the invention, radiant output is greater by almost a full degree of magnitude with only 20 watts of microwave power [curve (a)]. 1

The following table gives the optimum lamp operating temperatures and the observed stability characteristics utilizing the apparatus of FIGS. 1 and 2.

Also optimum temperature for maximum spectral radiance.

' Net source draft hr. measured over hr period.

Difference between maximum and minimum signals recorded over 15 hr period. v W I Peak-to-peak noise is percentage of spectral radiant output. Time constant of 0.5 sec was used.

The substantial effect of change in temperature of the EDL on the spectral radiant output thereof is to be seen in the chart of FIG. 5, and it will be noted that at temperatures beyond the optimum temperature for the specific metal, the output is reduced. This suggests the possibility of putting two or more excitable elements (or halides) in one envelope and selecting'the element which is to provide the desired spectral line by merely changing the temperature of the EDL. Experiments with such EDLs have shown that this is, at least for some purposes, a practical procedure.

Specifically, cadmium metal, zinc metal and mercury metal within an EDL exhibit the described effects, with useful spectral radiance of the characteristic lines of these respective metals being produced when the temperature of the EDL is maintained at the optimum temperature of a selected one of the metals, with attenuation of the radiance at the lines characteristic of the others. An EDL containing gallium iodide, indium iodide and thallium iodide may be similarly operated to select, by change in EDL temperature, the line of any one thereof as the major component of the radiance, and thorium, uranium and zirconium present in an EDL provide radiance lines similarly selectable.

The following Table 2 discloses results experimentally obtained of detection limits by atomic fluorescense utilizing the apparatus of FIG. 1 but with only partially optimized atomic fluorecence conditions as compared to the detection limits previously reported for EDLs employed with the most highly optimized conditions obtainable but without temperature control as shown herein. For example, in obtaining the test results reported, the test sample was atomized using an unshielded air/H flame rather than a shielded Ar/O /I-l flame, the latter being capable of much more precision, and having been performed without mirrors such as are normally employed to obtain the lowest detection limits by increasing the atomic fluorescence signal.

TABLE 2 Precision Pre- Useful (perviously analytical cent lowest range a standreported Wave- (,ug ml- Detection ard detection Elelength limit devialimits ment (nm) Lower Upper (pg ml tion) (pg/ml) Hg 253.7 1X10 1X10 2 X 10- 3.0 2 X 10- Cd 228.8 1X10" 1X10- 2 X 10* 2.1 1X10 Tl 377.6 1X10 IX 10- 1X10 3.5 8 X 10" Zn 213.8 1X10 1X10 2 X 10" 0.75 2 X10- Cu 324.7 1X10" 1.5 X10 1X10- 4.6 1X 10- Fe 248.3 1X10" 5 X10 2 X10 5.2 1.6 X10 Useful range results in precision or order of that shown in Column 6 of the table.

' Defined on basis of S/N 2.

From 20 observations.

Using flame atomizers and monochromating detectors. All values converted to S/N 2.

It is estimated that each detection limit obtained while using the source according to the invention but with relatively unsophisticated flame and optics in conjunction therewith could be reduced by an order of magnitude if all experimental conditions were fully optimized. It is noted that the detection limits for copper and mercury are lower by 60 times and 5 times respectively than the lowest previously reported detection limits using EDL spectral sources.

A copper EDL operated in the arrangement shown in FIG. 1 demonstrated very little metallic deposit on the inside of the quartz envelope even after running at maximum emission output for approximately hours, and there was no difficulty in obtaining a reproducible output from this source over that period. In contrast copper EDL sources operated in unheated systems of the prior art resulted in substantial metallic deposit after only a small number of hours and the output has tended to vary substantially with time of operation and has tended to vary upon each re-initiation.

While the invention has been described with respect to a certain specific embodiment, it will be appreciated that many modifications and changes may be made by those skilled in the art without departing from the spirit of the invention. It is intended, therefore, by the appended claims to cover all such modifications and changes as fall within the true spirit and scope of the invention.

What is claimed as new and what it is desired to secure by Letters Patent of the United States is:

l. A temperature stabilized spectral source comprising an electrodeless discharge lamp of the type including a hollow non-conductive lamp body enclosing a volatile excitable substance, a jacket of electrically nonconductive material spaced outwardly around said body having openings through which gas may enter into and flow out of said jacket, means for introducing a stream of gas through one of said openings into and through said jacket around said lamp body therein and out through another of said openings, a microwave energy source including an antenna providing a radiated microwave field at said lamp for exciting said substance therein, heat transfer means for heating the gas in said stream before introduction thereof into said jacket, temperature responsive means in said gas stream, and means for heating said heat transfer means, said temperature responsive means being connected to control said heating means.

2. The combination according to claim 1 wherein said jacket is an open ended quartz cylinder.

3. The combination according to claim 1 wherein said jacket comprises an open ended cylinder provided with a wall opening between its ends through which light from said lamp passes.

4. The combination according to claim 1 wherein said antenna is an A-type antenna which is disposed adjacent one of said open ends of said cylinder and oriented to focus said microwave field on said lamp.

5. A temperature stabilized spectral source comprising an electrodeless discharge lamp of the type including a hollow non-conductive lamp body enclosing a volatile excitable substance, an electrically nonconductive jacket having electrically non-conductive walls spaced outwardly around said body and having gas inlet and outlet openings, means for moving gas through said inlet and outlet openings to flow through said jacket and around said lamp body therein, a microwave energy source including a radiating antenna providing a microwave field at said lamp for exciting said substance, and means for controlling the temperature of the gas in said jacket.

6. The combination according to claim 5 wherein said means for controlling the temperature includes heating means remote from said jacket for heating the gas supplied to said inlet opening, and temperature responsive means exposed to the gas after it has passed said heating means controllingly connected to said heating means.

7. The combination according to claim 6 wherein said temperature responsive means is disposed in the path of the gas moving from said heating means toward said lamp body.

8. A temperature stabilized spectral source comprising an electrodeless discharge lamp of the type including a hollow, electrically non-conductive lamp body enclosing a volatile excitable substance, an elongated electrically non-conductive jacket having electrically non-conductive walls spaced outwardly around said body and having end openings, a gas duct connected to said jacket at one of said end openings and extending away from said jacket, fan means for moving a stream of gas through said duct and into said jacket through said one end opening and out of said jacket through the other end opening, a microwave energy source including an A-type antenna oriented and disposed to radiate a microwave field toward said lamp for exciting said substance, heating means disposed in said duct remote from said lamp for raising the temperature of the gas passing toward said jacket, temperature responsive means exposed to the heated gas entering said jacket, and means comprising said temperature responsive means for controlling said heating means.

9. The combination according to claim 8 wherein said jacket comprises an open ended quartz cylinder, said fan means is located at an end of said duct remote from said jacket and upstream of said heating means, and said temperature responsive means is disposed in said duct between said heating means and said one end opening of said jacket.

10. Apparatus for performing atomic absorption or atomic fluorescence analysis comprising a hollow nonconductive lamp body containing at least one volatile excitable substance generating a characteristic wavelength of radiant energy when excited and having an optimum operating temperature, an electromagnetic microwave energy source including an A-type antenna radiating a microwave field at said lamp body to excite said substance, means to direct a stream of gas around and in heat-exchange relationship with said lamp body, means to heat said gas stream, and thermostatic control means exposed to said gas stream and connected to said heating means adjusted to cause said heating means to heat said gas to a temperature which will maintain said optimum operating temperature of said lamp body to cause said substance to be excited to optimum emission at said characteristic wavelength.

11. The apparatus of claim 10, and wherein the excitable substance comprises a metal.

12. The apparatus of claim 10, and wherein the excitable substance comprises a metal halide.

13. The apparatus of claim 10, and wherein said means for directing the stream of gas comprises a quartz jacket surrounding said lamp body and means for moving the stream through said jacket, and wherein said heating means is disposed spacedly upstream from said jacket.

14. The apparatus of claim 13, and wherein said heating means comprises a heater coil, and said thermostatic control means comprises means to deliver a regulated current to said heater coil in accordance with variations of the temperature of the gas downstream from said coil and upstream of said lamp body.

i i I 

1. A temperature stabilized spectral source comprising an electrodeless discharge lamp of the type including a hollow nonconductive lamp body enclosing a volatile excitable substance, a jacket of electrically non-conductive material spaced outwardly around said body having openings through which gas may enter into and flow out of said jacket, means for introducing a stream of gas through one of said openings into and through said jacket around said lamp body therein and out through another of said openings, a microwave energy source including an antenna providing a radiated microwave field at said lamp for exciting said substance therein, heat transfer means for heating the gas in said stream before introduction thereof into said jacket, temperature responsive means in said gas stream, and means for heating said heat transfer means, said temperature responsive means being connected to control said heating means.
 2. The combination according to claim 1 wherein said jacket is an open ended quartz cylinder.
 3. The combination according to claim 1 wherein said jacket comprises an open ended cylinder provided with a wall opening between its ends through which light from said lamp passes.
 4. The combination according to claim 1 wherein said antenna is an A-type antenna which is disposed adjacent one of said open ends of said cylinder and oriented to focus said microwave field on said lamp.
 5. A temperature stabilized spectral source comprising an electrodeless discharge lamp of the type including a hollow non-conductive lamp body enclosing a volatile excitable substance, an electrically non-conductive jacket having electrically non-conductive walls spaced outwardly around said body and having gas inlet and outlet openings, means for moving gas through said inlet and outlet openings to flow through said jacket and around said lamp body therein, a microwave energy source including a radiating antenna providing a microwave field at said lamp for exciting said substance, and means for controlling the temperature of the gas in said jacket.
 6. The combination according to claim 5 wherein said means for controlling the temperature includes heating means remote from said jacket for heating the gas supplied to said inlet opening, and temperature responsive means exposed to the gas after it has passed said heating means controllingly connected to said heating means.
 7. The combination according to claim 6 wherein said temperature responsive means is disposed in the path of the gas moving from said heating means toward said lamp body.
 8. A temperature stabilized spectral source comprising an electrodeless discharge lamp of the type including a hollow, electrically non-conductive lamp body enclosing a volatile excitable substance, an elongated electrically non-conductive jacket having electrically non-conductive walls spaced outwardly around said body and having end openings, a gas duct connected to said jacket at one of said end openings and extending away from said jacket, fan means for moving a stream of gas through said duct and into said jacket through said one end opening and out of said jacket through the other end opening, a microwave energy source including an A-type antenna oriented and disposed to radiate a microwave field toward said lamp for exciting said substance, heating means disposed in said duct remote from said lamp for raising the temperature of the gas passing toward said jacket, temperature responsive means exposed to the heated gas entering said jacket, and means comprising said temperature responsive means for controlling said heating means.
 9. The combination according to claim 8 wherein said jacket comprises an open ended quartz cylinder, said fan means is located at an end of said duct remote from said jacket and upstream of said heating means, and said temperature responsive means is disposed in said duct between said heating means and said one end opening of said jacket.
 10. Apparatus for performing atomic absorption or atomic fluorescence analysis comprising a hollow non-conductive lamp body containing at least one volatile excitable substance generating a characteristic wavelength of radiant energy when excited and having an optimum operating temperature, an electromagnetic microwave energy source including an A-type antenna radiating a microwave field at said lamp body to excite said substance, means to direct a stream of gas around and in heat-exchange relationship with said lamp body, means to heat said gas stream, and thermostatic control means exposed to said gas stream and connected to said heating means adjusted to cause said heating means to heat said gas to a temperature which will maintain said optimum operating temperature of said lamp body to cause said substance to be excited to optimum emission at said characteristic wavelength.
 11. The apparatus of claim 10, and wherein the excitable substance comprises a metal.
 12. The apparatus of claim 10, and wherein the excitable substance comprises a metal halide.
 13. The apparatus of claim 10, and wherein said means for directing the stream of gas comprises a quartz jacket surrounding said lamp body and means for moving the stream through said jacket, and wherein said heating means is disposed spacedly upstream from said jacket.
 14. The apparatus of claim 13, and wherein said heating means comprises a heater coil, and said thermostatic control means comprises means to deliver a regulated current to said heater coil in accordance with variations of the temperature of the gas downstream from said coil and upstream of said lamp body. 